Devices and methods for bilateral motion of a load are known which utilize a rotating lead screw and a threaded nut that is driven by the lead screw. In U.S. Pat. No. 6,422,101, issued Jul. 23, 2002 to the present inventors, the entire teachings of which are incorporated herein by reference, a lead screw assembly is described which employs an anti-backlash nut assembly on a rotating lead screw. A hollow, elongated reinforcing rail extends lengthwise of and surrounds the lead screw. The reinforcing rail has a slot extending lengthwise of the central axis of the lead screw. The anti-backlash nut assembly has a nut which completely surrounds the outer diameter of the reinforcing rail, and is movable along the length of the reinforcing rail. The anti-backlash nut has a pair of threaded tongue portions which extend radially through the slot to engage the threads of the lead screw for moving the nut in reciprocating motion lengthwise of the rail. A load or tool can be mounted to the nut for reciprocating motion along the length of the assembly.
A lead screw assembly such as described in U.S. Pat. No. 6,422,101 is particularly advantageous for applications in which a high degree of spacial resolution in the reciprocating motion of a load is desired. However, the operation of any lead screw device is limited by the “critical speed” of the screw shaft. Beyond a certain critical speed, the rotation of the screw within the reinforcing rail becomes unstable. This critical speed is a function of both the length of the assembly and the diameter of the screw shaft. Thus, in the case of a four foot shaft with a ⅜ inch outer diameter, for instance, the “critical speed” is approximately 680 revolutions-per-minute (RPM). Beyond this speed, whipping and vibration forces become excessive, and the apparatus can self-destruct. In practice, this phenomena generally restricts the operation of the lead screw assembly to lower speeds over relatively short spans. It would be desirable to provide a lead screw assembly that can successfully reciprocate a load at high speeds over relatively long distances.